1. Field of the Invention
The present invention relates to a printer head used in a thermal ink-jet line printer and the like, a printer having the printer head, and a driving method for the printer head.
2. Description of the Related Art
FIG. 11 shows an example of a printer head in a known thermal ink-jet line printer. In the line printer, since one line is printed on a printing object at a time, a plurality of head chips 1 (1A, 1B, . . . ) are arranged side by side in the printing line direction. While only two head chips 1A and 1B are shown in FIG. 11, the plurality of head chips 1 are arranged side by side in the right and left direction of the figure.
The adjoining head chips 1 are placed offset from each other in the vertical direction. This is because an ink channel is formed between the upper head chip 1A and the lower head chip 1B in FIG. 11. These upper and lower head chips 1A and 1B perform discharging while shifting the discharge timing so that printed dots are arranged in a line.
Each head chip 1 has a plurality of discharging portions. The discharging portions are aligned in the printing line direction, and are arranged at predetermined intervals, as shown in FIG. 11. In the example shown in FIG. 11, the interval between the discharging portions is L. This also applies to all the head chips 1.
As shown in FIG. 11, the right-end discharging portion of the head chip 1A and the left-end discharging portion of the head chip 1B which adjoins the head chip 1A are placed with an interval L therebetween in the printing line direction. This allows all the ink droplets to land on a printing object at the intervals L even when the ink droplets are printed by using a plurality of head chips 1.
However, ink does not land on the initially designed positions due to the positional accuracy of the head chips 1, the positional accuracy for mounting heaters (not shown) which heat and discharge ink droplets, the positional accuracy of nozzles 2, or the like. In particular, the characteristics may greatly vary among the head chips 1. For this reason, the pitch between ink droplets which land on a printing object varies among the head chips 1.
This problem is marked particularly when the position of the heater and the position of the nozzle 2 are offset from each other. While the influence of the offset on the landing position varies depending on the structure of the discharging portion and the like, even when the center position of the heater and the center position of the nozzle 2 are offset by only 1 μm, the discharging direction is sometimes tilted 0.2 degrees.
In this case, when the discharging portion and the printing object are placed with a gap of 2 mm therebetween, the dot landing position is displaced by 7 μm from the normal position. Therefore, for example, even when the heaters are placed at the normal positions, and the positions of the nozzles 2 are displaced by −1 μm from the normal positions in the direction of arrangement of the discharging portions in one head chip 1, and are displaced by +1 μm from the normal positions in the direction of arrangement of the discharging portions on the other head chip 1, the landing position on the printing object at a distance of 2 mm from the discharging portion is displaced by −7 μm from the normal position in one head chip, and is displaced by +7 μm in the other head chip. Therefore, the interval is increased to a total of 14 μm.
FIGS. 12A to 12C show states in which ink droplets are discharged onto the printing object. In these figures, black circles in the left half represent ink droplets printed by the head chip 1A, and white circles in the right half represent ink droplets printed by the head chip 1B.
FIG. 12A shows a state in which there is no relative difference in landing position between the head chips 1A and 1B. In the case shown in FIG. 12A, the interval between the landing position of the right-end ink droplet from the head chip 1A and the landing position of the left-end ink droplet from the head chip 1B is substantially equal to the interval L of the ink-droplet landing positions in each head chip 1, and banding does not occur at the boundary therebetween.
In contrast, FIGS. 12B and 12C show examples in which there is a relative difference in landing position between the head chips 1A and 1B. FIG. 12B shows a state in which the landing interval between the head chips 1A and 1B is longer than L, and FIG. 12C shows a state in which the landing interval between the head chips 1A and 1B is shorter than L.
Consequently, the relative difference in landing position between the head chips 1A and 1B appears as a white band in FIG. 12B, and as a black band in FIG. 12C.
In order to prevent such differences in landing position between the head chips 1, the mounting accuracy of the nozzles 2 and the heaters is increased. However, there are limitations on increasing the accuracy.